The reminder window starts with "Reminders: Clicking on the title toggles details for item". In reality, what happens is that when an incidence's line is clicked, whether in the title column or in other columns, details toggle or not, depending on whether incidence details are showing, what incidence's details are showing, and what incidence is clicked.

I find it a strange behavior to hide details in any case, but there are several cases where this really makes no sense, for instance:
A. Double-clicking an incidence toggles details, then toggles again to the initial state. Note that opening an incidence with the Enter key does not work.
B. Assuming the window is showing incidence #1 above incidence #2, with the details hidden and incidence #1 selected, if one adds incidence #2 to the selection by Shift-clicking incidence #2, the details for incidence #1 appear. This does not happen if adding only with the keyboard (Shift + Down arrow).
C. With the same initial situation as in case B, if one moves the selection to incidence #2 using the Down arrow, the displayed details are not updated to incidence #2's details (while they are if one clicks to do the same).

This is worsened by a resizing issue: when details are shown, the window's height increases so details have a chance to fit, and correspondingly, when details are hidden, the window's height diminishes. However, clicking twice to toggle one way then the other is not idempotent - the window generally ends up smaller than in the initial situation. With Debian 9's KOrganizer, there is apparently some inferior limit, so the window always keeps a certain size, but in Kubuntu 19.04 (KOrganizer 5.10.3), the size always lowers, until even one incidence line no longer fits. This can be reproduced quickly by simply clicking repeatedly on the first incidence's row.

SOFTWARE/OS VERSIONS

Linux: 5.0.0
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2